Lactic acid is a very important organic acid and its applications are broad including food additive such as food preservative, condiment or acidifier, and industrial fields such as cosmetics, chemistry, metals, electronics, fabrics, dyeing textiles, and pharmaceutical industries. In addition, lactic acid is an essential ingredient of polylactic acid, one of biodegradable plastics. In recent years, there have been several concerns such as gas price increase, oil depletion and non-degradable petro-chemical based plastics. These concerns turn people's interests to environmentally-friendly polymer materials against recalcitrant non-biodegradable plastics which are main causes of environmental contamination. Accordingly, demand for lactic acid has been increased markedly. Worldwide production of lactic acid is approximately 100,000 tons every year, posting about 5% of growth. Considering the gradual increase in the biodegradable plastics, the demand for lactic acid will rise even higher within several years. Particularly, lactic acid is a very reactive organic acid containing a hydroxyl group and a carboxylic group. It is also used as a key ingredient of many chemicals including polylactic acid, acetaldehyde, polypropylene glycol, acrylic acid, and 2,3-pentathione. Moreover, lactic acid is used for preparation of ethyl lactate that is a biodegradable and non-toxic solvent widely used in manufacture of electronic instruments, paints or fabrics, detergents, adhesives or printings.
As described above, lactic acid is a very useful and valuable industrial material. Typically, lactic acid was produced through the traditional chemical synthesis or the biological fermentation process using carbohydrates as a substrate. The latter is preferred in many cases because the chemical synthesis creates other problems in addition to the material cost increase caused by the gas price increase or environmental contamination problems. For example, the traditional chemical synthesis of lactic acid not only produces lactic acid but also an inactive D, L lactic acid in form of a racemic mixture that consists of equal amount of D-lactic acid and L-lactic acid. Unfortunately though, the composition ratio of the D-lactic acid and the L-lactic acid cannot be controlled. Therefore, when the racemic lactic acid is used for preparing polylactic acid an amorphous polymer with low fusing point is produced, which in turn becomes an obstacle to the broad range of applications. On the other hand, the biological fermentation process using microorganisms makes it possible to selectively produce D-lactic acid or L-lactic acid depending on the strain used. For example, microorganisms such as Lactobacillus, Bacillus, Rhizopus, Streptococcus, and Enterococcus usually produce L-lactic acid. Microorganisms such as Leuconostoc and Lactobacillus vulgaricus usually produce D-lactic acid. The L-lactic acid is preferably used in cosmetic or food industries because the D-lactic acid is not metabolized in the body. Pure L-lactic acid can be used for the preparation of polylactic acid. In doing so, the polylactic acid is transformed into a crystal form having 180° C. of fusion point. Also, physical properties of the polylactic acid can be controlled by adjusting the content of the D-lactic acid for the polymerization. Overall, the L-lactic acid is applicable in replacement of various kinds of plastics.
As described above, lactic acid has a great potential to be an environmentally-friendly replacement material in a wide range of industrial applications. Thus, it is very important to develop a method for producing lactic acid through the biological fermentation which is more economical than the traditional chemical synthesis. To this end, there is an urgent need to develop a lactic acid-producing strain and a cost-effective method for producing lactic acid.